Communication System

ABSTRACT

The present application relates to a wireless communication system and related methods and apparatuses for transmitting a signal from a source apparatus to a destination apparatus, via at least one intermediate apparatus. In particular, the present invention relates to techniques which seek to improve the throughput of data in multi-hop communication systems.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.11/454,028, filed Jun. 16, 2006, the contents of which are herebyincorporated by reference.

FIELD

The present invention relates to a wireless communication system andrelated methods for transmitting a signal from a source apparatus to adestination apparatus, via at least one intermediate apparatus. Inparticular, the present invention relates to techniques which seek toimprove the throughput of data in multi-hop communication systems.

BACKGROUND

It is known that the occurrence of propagation loss, or “pathloss”, dueto the scattering or absorption of a radio communication as it travelsthrough space, causes the strength of a signal to diminish. Factorswhich influence the pathloss between a transmitter and a receiverinclude: transmitter antenna height, receiver antenna height, carrierfrequency, clutter type (urban, sub-urban, rural), details of morphologysuch as height, density, separation, terrain type (hilly, flat). Thepathloss L (dB) between a transmitter and a receiver can be modelled by:

L=b+10n log d   (A)

Where d (metres) is the transmitter-receiver separation, b(db) and n arethe pathloss parameters and the absolute pathloss is given by 1=1()wit))

FIG. 1A illustrates a single-cell two-hop wireless communication systemcomprising a base station (known in the context of 3G communicationsystems as “node-B” (NB)) a relay node (RN) and a user equipment (UE).In the case where signals are being transmitted on the downlink (DL)from a base station to a destination user equipment (UE) via the relaynode (RN), the base station comprises the source apparatus (S) and theuser equipment comprises the destination apparatus (D). In the casewhere communication signals are being transmitted on the uplink (UL)from user equipment (UE), via the relay node, to the base station, theuser equipment comprises the source apparatus and the base stationcomprises the destination apparatus. The relay node is an example of anintermediate apparatus (I) and comprises: a receiver, operable toreceive a signal from the source apparatus; and a transmitter, operableto transmit this signal, or a derivative thereof, to the destinationapparatus.

Table I below gives some examples of the calculated pathloss of a signalbeing transmitted over the different links: source to destination (SD),source to intermediate (SI) and intermediate to destination (ID), in amulti-hop transmission system where b and n are assumed to remain thesame over each of the links.

TABLE I Separation (metres) Pathloss in dB Absolute Pathloss b(dB) n SDSI ID SD SI ID SD SI ID 15.3 3.76 1000 500 500 128.1 116.8 116.8 6.46E124.77E11 4.77E11 15.3 3.76 1000 600 600 128.1 119.76 119.76 6.46E129.46E11 9.46E11 15.3 3.76 1000 700 700 128.1 122.28 122.28 6.46E121.69E12 1.69E12

The examples calculated above demonstrate that the sum of the absolutepath losses experienced over the indirect link SI+ID may be less thanthe pathloss experienced over the direct link SD. In other words it ispossible for:

L(SI)+L(ID)<L(SD)   (B)

Splitting a single transmission link into two shorter transmissionsegments therefore exploits the non-linear relationship between pathlossverses distance. From a simple theoretical analysis of the pathlossusing equation (A), it can be appreciated that a reduction in theoverall pathloss (and therefore an improvement, or gain, in signalstrength and thus data throughput) should be achieved if a signal issent from a source apparatus to a destination apparatus via anintermediate apparatus (eg relay node), rather than being sent directlyfrom the source apparatus to the destination apparatus. If implemented,multi-hop communication systems could potentially allow for a reductionin the transmit power of transmitters which facilitate wirelesstransmissions, which would lead to a reduction in interference levels aswell as decreasing exposure to electromagnetic emissions.

Clearly, due to the non-linear relationship between pathloss anddistance, the position of an intermediate apparatus relative to thesource and destination, will critically effect the potential gain that amulti-hop transmission may have as compared to a direct, or single-hop,transmission between the source and destination. This is illustrated inFIG. 2A which shows a graphical representation of the theoretical gainwhich may be achieved by multi-hop transmissions, and plots the totalpower loss (dB) against the relative normalised position of theintermediate apparatus between the source apparatus and the destinationapparatus.

Considering firstly the case where the intermediate node is positionedon the line of the direct link between the source and destination (inwhich case the path extension factor (s)=1), it can be seen that thepotential gain is reduced as the relay node is moved away from a mid-wayposition towards the source or destination apparatus. Likewise, as theposition of the intermediate apparatus is moved away from the line ofthe direct link, thereby extending the total path length of the sum ofthe two transmission segments (and increasing the path extension factorto s=1.1, s=1.2 etc), it can be seen that the graphical region oftheoretical gain is again reduced.

However, simulations carried out to test the applicability of multi-hopcommunication systems have revealed unexpectedly low gains in throughputof data. Indeed, the gains experienced are well below the potential gainsuggested by a simple analysis based on the pathloss equation A.Consequently, and despite the potential advantages that multi-hopsystems may demonstrate in terms of signal range extension, a possiblereduction in the overall transmit power required to transmit a signalbetween a source and destination, and the connectivity of otherwiseinaccessible nodes, wireless systems operators have been deterred fromimplementing multi-hop networks.

One of the reasons that such a discrepancy exists between the predictedgain and the simulated gain is that previous predictions have been basedon the assumption that the pathloss parameters b and n are the same onall links. In actual fact, these values vary as a result of the antennaheight of the source apparatus and destination apparatus as compared tothe height of the relay node. Thus, a more realistic table of values isgiven below in table II. The values labelled 3GPP are obtained fromadapting the model employed by the 3GPP to incorporate the fact that theantenna height of the intermediate apparatus is typically somewherebetween the height of the antenna at the source and destinationapparatus. The values labelled UoB are derived from modelling conductedby the University of Bristol based on a typical deployment in the cityof Bristol.

TABLE II Link Pathloss Parameter S-D S-I I-D 3GPP b (dB) 15.3 15.5 28 n3.76 3.68 4 UoB b (dB) 13.07 16.29 10.04 n 4.88 4.64 5.47

The graphical illustration of total pathloss verses normalised relaynode position using the pathloss parameters tabulated in table II isshown in FIG. 2B. It can be seen that the perfect “bell-shape” of FIG.2A is not achieved when a more realistic set of pathloss parameters areused to calculate the variation in total pathloss as the position of atheoretical relay node is adjusted. Indeed, the region of gain isreduced and it is apparent that relatively small changes in the positionof a relay node or a user equipment, leading to a change in the absolutepathloss over the communication link, will have a significant effect onthe quality of a communication signal at the receiving apparatus. Thus,the positioning of an intermediate apparatus or relay node is criticalif a gain is to be achieved by the occurrence of a multi-hoptransmission, as compared to a direct transmission between the sourceand destination.

However, even when predictions are based on a more accurate reflectionof the pathloss parameters likely to be encountered in the real world,simulations of multi-hop systems have revealed unexpectedly poorcorrespondence between the predicted and simulated gain.

Embodiments of the present invention seek to provide a communicationsystem comprising a source apparatus, a destination apparatus and atleast one intermediate apparatus, wherein the source apparatus and theor each intermediate apparatus each comprise a transmitter, operable totransmit a communication signal or a signal derived therefrom, in acommunication direction towards said destination apparatus, and whereinthe destination apparatus and the, or each, intermediate apparatus eachcomprise a receiver, operable to receive said communication signal, or asignal derived therefrom, wherein said communication system comprises adetermining means, operable to determine a measure of, or a change in ameasure of, the resource allocated to one or more of said transmittersthat will tend to substantially attain or maintain a balance between:

i) a measure of the quality of the communication signal received at thedestination apparatus; and

ii) measure of the quality of the communication signal received at the,or each, intermediate apparatus.

It will, of course, be appreciated that the communication signalactually received by the destination apparatus may be the communicationsignal transmitted by the source apparatus, or it may be a communicationsignal derived therefrom.

Thus, preferred embodiments of the present invention seek to maintain orachieve a “balance” in a measure of the quality of a communicationsignal being received at the or each intermediate apparatus and ameasure of the quality of a communication signal being received at adestination apparatus. Preferably, the determining means is operable todetermine a change in the transmit power of one or more of theapparatuses which are operable to transmit a communication signalpresent communication system embodying the present invention, in orderto reduce or prevent substantial imbalance (i.e. achieve or maintain asubstantial “balance”) between a measure of the quality of acommunication signal received at the intermediate apparatus and ameasure of the quality of a communication signal received at thedestination apparatus.

The existence of an imbalance arising in a communication systemembodying the present invention may be apparent from a direct comparisonof a measure of a quality of a communication signal received at thedestination apparatus and a measure of the quality of a communicationsignal received at the, or one of the, intermediate apparatuses.Alternatively, an imbalance may be apparent when a comparison is madevia a mapping function. Hence the situation may exist where measures ofequal value do not equate to a balanced system, and likewise wheremeasures of differing value may equate to a balanced system.

It is envisaged that embodiments of the present invention may be used,prior to deployment of a multi-hop system, to optimise the system and/orto substantially balance a measure of the quality of a communicationsignal received at the, or each intermediate apparatus and a measure ofthe quality of a communication signal received at the destinationapparatus. It is also envisaged that embodiments of the presentinvention may be implemented within an existing multi-hop system inorder to seek to achieve and maintain “balance” in a measure of thequality of a communication signal across all links. Thus, the presentinvention may be employed within a multi-hop communication system toestablish a substantial “balance” between an indicator of the RSS or theSINR at the destination apparatus and an indicator of the RSS or theSINR, at the, or each, intermediate apparatus. The transmit powers willadvantageously be optimised initially with respect to a target receivedsignal quality for one of the apparatuses operable to receive acommunication signal in a multi-hop system. This will usually be thedestination apparatus. Thus, an indicator of a measure of the variationof the quality of a communication signal received at the destinationfrom a target received signal quality (=“variation from target”indicator), will advantageously be minimal when a system has beenoptimised according to embodiments of the present invention. Thereafter,if a change is detected in the variation from target indicator, whichmay be in a positive or negative sense, e.g. if the quality of thecommunication signal has deteriorated or improved, or if the target setfor the apparatus has changed, the variation from target indicator willincrease. In this case, embodiments of the present invention whichenable a deviation of the variation from target indicator from a desiredvalue to be detected, will advantageously seek to bring the variationfrom target indicator to the desired value.

Simulations of multi-hop communication systems embodying the presentinvention have been found to demonstrate a significant gain over systemsin which a signal is transmitted directly to a destination apparatus.Indeed, the results of system level simulations carried out to test apreferred embodiment of the present invention indicate that acommunication system which is “balanced” within the context of thepresent invention, can be expected to fulfil the advantages associatedwith multi-hop transmissions and to provide an improvement in thethroughput of data.

It is believed that one explanation for the improved throughputdemonstrated by preferred embodiments of the present invention is thatthey permit a reduction in the absolute transmit power required in amulti-hop system. This is considered in more detail below.

Starting from the principle already demonstrated above, that bysplitting a single direct transmission link into two shortertransmission links, a reduction in the total pathloss experienced by asignal is achieved. Then, the total transmit power required to transmita communication signal from a source apparatus to a destinationapparatus via at least one intermediate apparatus, will be less than isrequired to transmit the communication signal directly between thesource apparatus and the destination apparatus. Thus, less transmitpower is needed in order to ensure that the destination apparatus (andpossibly also the intermediate apparatus) receives a minimum or “target”signal quality. If no adjustment is made to the transmit power, thensignificant excess transmit power (i.e. transmit power exceeding thatrequired to achieve a good, or target, signal quality at the destinationapparatus and/or the intermediate apparatus) will result. Rather thanserving to further increase the gain achieved by a multi-hopcommunication as compared to a direct communication between a sourceapparatus and a destination apparatus, this excess transmit power willmerely increase interference levels leading to a deterioration in thequality of the communication link. This deterioration will tend tocounteract the potential gain of a multi-hop system which accounts forthe poor simulation results of previously considered multi-hopcommunication systems.

Furthermore, the overall throughput across a two-hop network (forexample) is limited by the lower of: the number of data packets receivedat the intermediate apparatus and the number of data packets received atthe destination apparatus. The number of data packets received at areceiver is dependent upon the quality of the communication link thatterminates at that receiver. This may be reflected, for example, by ameasure of the throughput, a measure of the received signal strength(RSS) or a measure of the signal-to-interference plus noise ratio(SINR). Thus, in effect, the receiver which receives the lowest qualitycommunication signal within a multi-hop system forms a “bottle neck” fordata packet transmission, thereby wasting capacity for data transfer onother links within' the multi-hop system. An increase the transmit powerat a transmitter which does not serve to improve the lowest qualitycommunication signal, will result in additional excess transmit power.Consequently, a further degradation is experienced in the performance ofthe system. This is illustrated in FIGS. 9A and 9B which plot thevariation of the gain in average packet throughput observed by users ofa two-hop system compared to that observed for a single hop system,against the transmit power of the source apparatus (NB). Each graphincludes four different plots, each representing a different transmitpower of the intermediate apparatus. It can be seen that as the transmitpower of the base station is increased beyond an optimal point, then asignificant degradation in gain will be experienced despite the emissionof more signal energy.

It can therefore be appreciated that the improvements made by preferredembodiments of the present invention can be attributed to the way inwhich the various aspects of the present invention seek to ensure thatany imbalance between a measure of the quality of a communication signalreceived at the destination apparatus and a measure of the quality of acommunication signal received at the, or each, intermediate apparatus isreduced or prevented. Thus, excess transmit power which cannot improvethe throughput of data packets and which will only serve to raiseinterference levels, is minimised.

There are a number of different events which, if they arise, canpotentially lead to an “imbalance” (i.e. a difference between a measureof the quality of a communication signal received at the destinationapparatus and a measure of the quality of a communication signalreceived at the or each intermediate apparatus) in a multi-hop system:

-   -   i) The pathloss arising over one of the links changes. This may        be due to the position of one or both of the transmitter and        receiver for that link changing, or due to a change in the        environmental conditions or interference levels arising between        the transmitter and the receiver.    -   ii) It is usual for an apparatus which is operable to receive a        communication signal, to have a target RSS or target SINR. This        is usually set by the network provider and may vary depending on        the characteristics of the communication system or receiving        apparatus, or depending on the type of data to be transmitted.        The target RSS/SINR of a mobile phone or other user equipment        may vary and any change in target can be accommodated for by        adjusting the transmit power of the transmitting apparatus in        such a way as to tend to minimise a measure of the variation of        the quality of a communication signal received at the        destination apparatus from a target received signal quality        (i.e. “variation from target”). In the case of a multi-hop        system, merely adjusting the transmit power of one apparatus in        order to accommodate a change in target of one of the receiving        apparatuses, will lead to an imbalance within the system.

Embodiments of the present invention seek to provide a way of respondingto an imbalance, or a potential imbalance, which arises as a result ofeach of these possible events in order to improve the throughput of databeing transmitted on the uplink (UL) from a source apparatus to a basestation via one or more intermediate apparatuses. In a standardcommunications system the uplink is the link between the UE and the NB.In the multi-hop case the UL refers to the link in which communicationis directed towards the NB (e.g. UE to RN, RN to RN in the direction ofNB and RN to NB) . Furthermore, embodiments of the present inventionseek to provide a way of optimising a multi-hop system whereby anytarget quality set by one or more of the receivers is substantiallyattained and the throughput of data across each link is substantiallyequal.

According to a first aspect of the present invention there is provided acommunication system comprising a source apparatus, an intermediateapparatus and a base station, the source apparatus being operable totransmit a communication signal, via the or each intermediate apparatus,to the base station, the base station comprising indicator derivationmeans operable to derive one or more indicators of the quality of acommunication signal received at the base station, and wherein thecommunication system further comprises:

-   -   i) control means provided in the base station;    -   ii) indicator deviation detection means operable to detect a        change in one said indicator derived by the base station from a        desired value;    -   iii) determining means operable, following the detection of such        a change, to determine a required change in the transmit power        of the intermediate apparatus that will tend to bring said        indicator to said desired value, wherein said determining means        further comprises request transmitting means operable to        transmit a request for a change in the transmit power of the        intermediate apparatus to the control means.

Embodiments of the first aspect of the present invention advantageouslyprovide a way of responding to a deviation in the indicators derived bythe base station from a desired value which may be due to i) a change inpathloss between the intermediate apparatus and the base station; or ii)a change in the target of the base station by calculating a new transmitpower for the intermediate apparatus and the source apparatus.Advantageously, the change in the transmit power that is required willbe relative to the degree of deviation detected by the indicatordeviation detection means.

In accordance with an embodiment of the first aspect of the presentinvention, one of the indicators derived by said base station maycomprises a measure of the strength of a communication signal receivedat the base station (e.g. RSS). Preferably however, one of theindicators derived by the base station comprises a measure of thesignal-to-interference plus noise ratio (SINR) of a communication signalreceived at the base station, and/or it may comprise a measure of thevariation of the quality of a communication signal received at the basestation from a target received signal quality set for the base station.An indicator of the variation from target may be a variation from targetRSS, a variation from target SINR or a variation from a target which isbased on a combination of RSS and SINR. If a variation from targetindicator derived by the base station changes, embodiments of the firstaspect of the present invention will seek to bring the variation fromtarget indicator to the desired value.

Preferably the control means is operable, following receipt of'a requestfor a change in the transmit power of the intermediate apparatus, toissue a command, to said intermediate apparatus, commanding a change thetransmit power of the intermediate apparatus. The control means mayadvantageously comprises input signal receiving means, operable toreceive an input signal which allows the control means to determine ifan increase in transmit power of the intermediate apparatus isprohibited. Therefore, if the required change in transmit power of theintermediate apparatus comprises an increase in transmit power, andfollowing a determination by the control means that an increase intransmit power of the intermediate apparatus is prohibited, the controlmeans is operable to ignore said request. However, if the requiredchange in transmit power of the intermediate apparatus comprises anincrease in transmit power, and following a determination by the controlmeans that an increase in transmit power of the intermediate apparatusis not prohibited, the control means is operable issue a command to theintermediate apparatus, commanding a change in transmit power of theintermediate apparatus. The intermediate apparatus preferably comprisescommand receiving means operable to receive such a command from thecontrol means of the base station. According to a preferred embodimentsaid intermediate apparatus is operable to determine, based on a maximumtransmit power of the intermediate apparatus, if the intermediateapparatus can carry out the change in transmit power according to thecommand. Then, if the intermediate apparatus determines that it cannotcarry out the change in transmit power according to said command, saidintermediate apparatus is operable to determine a revised change intransmit power of the intermediate apparatus which can be carried out bysaid intermediate apparatus. The intermediate apparatus is operable tocause the transmit power of the intermediate apparatus to change inaccordance with said request, or said revised request, as the case maybe.

According to a second aspect of the present invention there is provideda communication system comprising a source apparatus, an intermediateapparatus and a base station, the source apparatus being operable totransmit a communication signal, via the or each intermediate apparatus,to the base station, the wherein each of the base station and theintermediate apparatus comprise: indicator derivation means operable toderive one or more indicators of the quality of a communication signalreceived at the base station or the intermediate apparatus respectively,the communication system further comprising:

-   -   i) imbalance detection means operable to detect an imbalance        between one said indicator derived by the base station and one        said indicator derived intermediate apparatus; and    -   ii) determining means operable, following detection of such an        imbalance by said imbalance detection means, to determine a        required change in the transmit power of the source apparatus        that will tend to reduce such an imbalance; and    -   iii) control means provided in said base station and operable,        following determination of said change, to issue a command to        said source apparatus commanding a change in the transmit power        of the source apparatus.

Embodiments of the second aspect of the present invention advantageouslyprovide a way of adjusting the transmit power of the source apparatus inorder to substantially restore a balance between a measure of a qualityof a communication signal received at the base station and a measure ofthe quality of a communication signal received at the intermediateapparatus. The imbalance may be due to a change in pathloss between thesource apparatus and the intermediate apparatus. Alternatively animbalance may arise following operation by a communication systemembodying the first aspect of the present invention to respond to achange in the target quality indicator of the base station, since inrestoring the variation from target indicator to its original measure(by changing the transmit power of the intermediate apparatus),the-quality indictors of the intermediate apparatus and the base stationapparatus will no longer be balanced.

According to embodiments of the second aspect of the present invention,one said indicator derived by each of the intermediate apparatus and thebase station comprises a measure of the strength of a communicationsignal received at the base station or the intermediate apparatusrespectively (eg RSS). Preferably however, one said indicator derived byeach of said intermediate apparatus and said base station comprises ameasure of the signal-to-interference plus noise ratio (SINR) of acommunication signal received at the base station or the intermediateapparatus respectively.

Preferably the source apparatus is operable, following receipt of acommand, and wherein said command is for an increase in transmit power,to determine if it can carry out the command based on the maximumtransmit power of the source apparatus. If said source apparatusdetermines that it cannot carry out the said command, the sourceapparatus is operable to determine a revised change in transmit powerthat will tend to reduce the said imbalance, and to carry out saidrevised change. Furthermore, the control means is preferably operable,following issuance of a command to the source apparatus and wherein saidcommand was for an increase in transmit power, to monitor the indicatorderived by the intermediate apparatus in order to determine if saidtransmit power of said source apparatus has been changed in accordancewith said command. If it is determined that a change in transmit powerof the source apparatus has not been carried out in accordance with saidrequest, the control means is operable to prohibit any subsequentincreases in transmit power of said intermediate apparatus. If anincrease in transmit power of said intermediate apparatus is prohibited,and if no subsequent imbalance is detected by said imbalance detectionmeans, said control means is then operable to allow a subsequentincrease in transmit power of said intermediate apparatus. If anincrease in transmit power of said intermediate apparatus is prohibited,and if a subsequent imbalance is detected by said imbalance detectionmeans such that said control means of said base station is caused toissue a command to said source apparatus and wherein said command is fora decrease in transmit power, said control means is operable to allow asubsequent increase in transmit power of said intermediate apparatus.Furthermore, if an increase in transmit power of said intermediateapparatus is prohibited, and if a subsequent imbalance is detected bysaid imbalance detection means such that said control means of said basestation is caused to issue a command to said source apparatus andwherein said instruction is for an increase in transmit power which canbe carried out by said source apparatus, said control means is operableto allow a subsequent increase in transmit power of said intermediateapparatus.

The intermediate apparatus preferably comprises a receiver operable toreceive the signal transmitted by the source apparatus; and atransmitter operable to transmit the received signal, or a signalderived therefrom, to the destination apparatus. Duplexing of signals toseparate communication signals received by the intermediate apparatusfrom communication signals transmitted by the intermediate apparatus maybe Frequency Division Duplex (FDD) or Time Division Duplex (TDD). One ormore of the intermediate apparatuses may preferably comprise a so-calledrelay node (RN) or relay-station (RS). A relay node has the capabilityof receiving a signal for which it is not the intended final destinationand then transmitting the signal on to another node such that itprogress towards the intended destination. A relay node may be of theregenerative type, where the received signal is decoded to the bitlevel, making a hard decision. If the received packet is found to be inerror then retransmission is requested, hence the RN incorporates ARQ orH-ARQ. QRQ or H-ARQ is a receiver technique for managing retransmissionrequest and subsequent reception of retransmitted signals. Once thepacket is successfully received, it is then scheduled for retransmissiontowards the destination, based on any radio resource managementstrategies incorporated into the RN. Alternatively a relay node may beof the non-regenerative type, whereby data is amplified at the relaynode and the signal is forwarded to the next station. It is envisagedthat the function of an intermediate apparatus or relay node may beprovided by a mobile phone, or other user equipment.

In accordance with embodiments of the first and second aspect of thepresent invention, which seek to adjust the quality indicators at theintermediate apparatus and the destination apparatus by adjusting thetransmit power of the base station and the intermediate apparatusrespectively and without explicitly calculating the transmit powers ofthe base station and intermediate apparatus, a relay node of theregenerative type is preferably used where the received signal isdecoded to the bit level, making a hard decision. A regenerative relaynode is operable to receive a communication signal transmitted by thesource apparatus and to decode the signal to bit level beforetransmitting a new signal to the next station in the multi-hop system(which may be the destination UE or another intermediate apparatus).

The desired aim of the present invention is to set the allocation ofresource at each transmitting apparatus such that the throughput acrosseach link in the multi-hop system is equal. As the throughput is afunction of the received SINR it follows that in order to balance thethroughput across the multi-hop links, the received SINR at each nodemust be balanced. In the case of regenerative relays the SINR at a givenapparatus is not a function of the SINR at any other node. This ofcourse assumes equal SINR performance at all nodes. Thus, it is possibleto ensure the required SINR which ensures that the system issubstantially balanced and that the target SINR at the destination ismet, can be achieved by simply adjusting the transmit power relative tothe difference between the actual and required SINR. Further if thetarget SINR changes at one apparatus, then it is possible to adjust thetransmit power at all nodes in a manner relative to the required change.Consequently, there is no need to calculate the actual transmit powerand the implementation of embodiments of the present invention isadvantageously computationally simple. It is envisaged that the functionof an intermediate apparatus or relay node may be provided by a mobilephone, or other user equipment.

Whilst embodiments of the present invention may only realisticallyoperate where regenerative relays are employed as the intermediateapparatus, they benefit from a relatively simple determination of thetransmit powers which does not require the performance of an explicitcalculation. Transmit powers are advantageously determined by adjustingthe transmit power of the relevant transmitter relative to the degree ofindicator change detected by the indicator deviation detection means, inorder to restore the indicator which has experienced a change to itsvalue, and thereby balance the received SINR.

Furthermore, embodiments of the present invention advantageously enablecentralised control of the setting of the transmit power to bemaintained, with minimal processing required in the relay station. Thisis beneficial to the operator of the wireless system as it keep controllocated within a central entity making management of the network muchsimpler. Further, should the relay start to malfunction, then due to thefact that control is located in the base station (or Node—B) thencorrective measures are possible by the operator. Moreover, the factthat processing in the intermediate apparatus is kept to a minimum isadvantageous in terms of reducing power consumption and thus maximisingbattery life, should the intermediate apparatus be a mobile or remotedevice.

The first and second aspects of the present invention will each tend toreduce or prevent an imbalance which arises or may arise, as the casemay be, under different circumstances. For example, the situation mayarise where the pathloss between the intermediate apparatus and the basestation changes or the target of the base station may change. Both ofthese events leads to a change in the indicator derived by the basestation and can advantageously be addressed by embodiments of the firstaspect of the present invention. Preferably, a communication systemembodying the first aspect of the present invention will comprise adeviation detection means which monitors the, or one of the, indicatorsof the destination apparatus at all times. Thus, any change or deviationin the indicator derived by the destination apparatus, can be detectedquickly.

The first aspect alone may be sufficient to restore balance across amulti-hop system following a change in the pathloss between theintermediate apparatus and the base station. However, as discussedabove, if the pathloss between the source apparatus and the intermediateapparatus changes (which may be due to a change in the position of theintermediate apparatus and/or the source apparatus or due to a change inthe environmental conditions arising between the intermediate apparatusand the source apparatus), this must be dealt with by embodiments of thesecond aspect of the present invention. Moreover, in order to restorebalance to a multi-hop communication system following a change in thetarget quality set by the destination apparatus, it is necessary forboth the transmit power of the intermediate apparatus and the sourceapparatus to be adjusted. Thus, in order to deal with a change in thetarget quality indicator of the destination apparatus, a communicationsystem which embodies both the first and second aspect of the presentinvention is preferably provided. Preferably, the imbalance detection ofthe second aspect of the present invention is performed periodically.Thus according to a preferred embodiment of the first aspect of thepresent invention said intermediate apparatus comprises indicatorderivation means operable to derive an indicator of the quality of acommunication signal received at the intermediate apparatus and whereinsaid intermediate apparatus is further operable to transmit saidindicator to an indicator receiving means of said base station, whereinsaid base station further comprises an imbalance detection meansoperable to detect an imbalance between an indicator derived by the basestation and an indicator derived by the intermediate apparatus, andwherein said determining means is further operable, following detectionof such an imbalance by said imbalance detection means, to determine arequired change in the transmit power of the source apparatus that willtend to reduce such an imbalance, wherein said control means is furtheroperable, following determination of said change, to issue a command tosaid source apparatus command a change in the transmit power of thesource apparatus in accordance said required change.

The source apparatus is preferably operable, following receipt of acommand, and wherein said command is for an increase in transmit power,to determine if it can carry out the command based on the maximumtransmit power of the source apparatus. If said source apparatusdetermines that it cannot carry out the said request, the sourceapparatus is operable to determine a revised change in transmit powerthat will tend to reduce the said imbalance, and to carry out saidrevised change.

Preferably the control means is operable, following issuance of acommand to the source apparatus and wherein said command was for anincrease in transmit power, to monitor said indicator derived by theintermediate apparatus in order to determine if said transmit power ofsaid source apparatus has been changed in accordance with said command.If it is determined that a change in transmit power of the sourceapparatus has not been carried out in accordance with said command, saidcontrol means is operable to prohibit any subsequent increases intransmit power of said intermediate apparatus. If an increase intransmit power of said intermediate apparatus is prohibited, and if nosubsequent imbalance is detected by the imbalance detection means, saidcontrol means is operable to allow a subsequent increase in transmitpower of the intermediate apparatus.

Preferably, if an increase in transmit power of said intermediateapparatus is prohibited, and if a subsequent imbalance is detected bysaid imbalance detection means such that said control means of said basestation is caused to issue a command to the source apparatus and whereinthe command is for a decrease in transmit power, the control means isoperable to allow a subsequent increase in transmit power of saidintermediate apparatus. If an increase in transmit power of saidintermediate apparatus is prohibited, and if a subsequent imbalance isdetected by the imbalance detection means such that the control means ofthe base station is caused to issue a command request to the sourceapparatus and wherein the command is for an increase in transmit powerwhich can be carried out by said source apparatus, said control means isoperable to allow a subsequent increase in transmit power of saidintermediate apparatus.

The situation may arise where a change in the target of the base stationapparatus is accommodated by a substantially simultaneous change in thepathloss between the intermediate apparatus and the base station. Thus,in this case no request for a change in transmit power of theintermediate apparatus will be generated by the base station. This,relatively rare, situation can be handled by a communication systemwhich embodies the second aspect of the present invention since a changein the measure of the pathloss experienced between the intermediateapparatus and the base station will give rise to an imbalance betweenthe signal quality indicators derived by the intermediate anddestination apparatuses and this imbalance will be detected by theimbalance detection means. The determining means is then operable todetermine the change in the transmit power of the base station that isrequired to in order to tend to reduce the imbalance between a measureof a quality of a communication signal received at the intermediateapparatus and a measure of the quality of a communication signalreceived at the base station.

According to an embodiment of the first aspect of the present inventionthere is provided a method of controlling the transmit power of anintermediate apparatus in a multi-hop communication system, thecommunication system comprising a source apparatus, a base station andat least one intermediate apparatus, the source apparatus being operableto transmit a communication signal, via the or each intermediateapparatus, to the base station, the method comprising the steps of:

-   -   i) deriving, at the base station, one or more indicators of a        quality of a communication signal received at the base station;    -   ii) detecting a deviation in the, or one of the, indicators        derived by the base station from a desired value; and    -   iii) determining a required change in the transmit power of the        intermediate apparatus that will tend to bring said indicator to        said desired value.

According to an embodiment of the second aspect of the present inventionthere is provided a method of controlling the transmit power of a sourceapparatus in a multi-hop communication system, the multi-hopcommunication system comprising a source apparatus, a base station andat least one intermediate apparatus, the source apparatus being operableto transmit a communication signal, via the or each intermediateapparatus, to the base station, the method comprising the steps of:

-   -   i) deriving, at each of the base station and the intermediate        apparatus, one or more indicator(s) of a quality of a        communication signal received at the base station, or at the        intermediate apparatus, respectively;    -   ii) detecting an imbalance between one said indicator derived by        the base station and one said indicator derived by the        intermediate apparatus;    -   iii) determining a required change in the transmit power of the        source apparatus that will tend to reduce such an imbalance; and    -   iv) issuing a command to said source apparatus commanding a        change in the transmit power of said source apparatus.

According to another embodiment of the first aspect of the presentinvention there is provided a base station operable to receive, via oneor more intermediate apparatus a communication signal from a sourceapparatus, the base station comprising:

-   -   i) indicator derivation means operable to derive one or more        indicators of a quality of a communication signal received at        the base station;    -   ii) indicator deviation detection means, operable to detect a        deviation in the, or one of the, indicators derived by the        indicator derivation means; from a desired value;    -   iii) determining means operable, following detection of such a        change by said indicator deviation detection means, to determine        a required change in the transmit power of the intermediate        apparatus that will tend to bring the said indicator towards        said first determining means being operable to transmit a        request for a change in the transmit power of the intermediate        apparatus to said control means;    -   iv) control means, operable to receive such a request from said        determining means.

The control means of the base station may advantageously furthercomprise input signal receiving means, operable to receive an inputsignal which allows the control means to determine if an increase intransmit power of the intermediate apparatus is prohibited.

Preferably, the base station may further comprise :

-   -   i) receiving means, operable to receive an indicator derived by        said intermediate apparatus, which indicator is indicative of a        quality of a communication signal received at the intermediate        apparatus;    -   ii) imbalance detection means operable to detect an imbalance        between an indicator derived by the base station and an        indicator received from the intermediate apparatus ;    -   wherein said determining means is operable, following detection        of such an imbalance by said imbalance detection means, to        determine a required change in the transmit power of the source        apparatus that will tend to reduce such an imbalance, said        control means being further operable, following determination of        said required change, to issue a command to said source        apparatus commanding a change in the transmit power of said        source apparatus.

According to another embodiment of the second aspect of the presentinvention there is provided a base station operable to receive, via oneor more intermediate apparatus, a communication signal from a sourceapparatus, the base station comprising:

-   -   i) indicator derivation means operable to derive one or more        indicator of the quality of a communication signal received at        the base station;    -   ii) indicator receiving means operable to receive one or more        indicator from the intermediate apparatus, wherein the or each        indicator is indicative of the quality of a communication signal        received at the intermediate apparatus;    -   iii) imbalance detection means operable to detect an imbalance        between an indicator derived by the base station and an        indicator received from the intermediate apparatus; and    -   iv) determining means operable, following detection of such an        imbalance by said imbalance detection means, to determine a        required change in the transmit power of the source apparatus        that will tend to reduce such an imbalance; and    -   v) said control operable, following determination of said        change, to issue a command to said source apparatus commanding a        change in the transmit power of said source apparatus.

Communication methods carried out in a base station embodying thepresent invention, an intermediate apparatus embodying the presentinvention or in a destination apparatus embodying the present inventionare also provided.

Embodiments of the present invention are particularly suited tostructured multi-hop systems which employ regenerative relays witheither TDD or FDD duplexing to separate communication signals receivedat the intermediate apparatus from signals transmitted from theintermediate apparatus.

The desired value may be the value of the indicator of the quality of acommunication signal derived by the destination apparatus which is at,or close to, the target value set by the destination apparatus, and whenthe system is substantially balanced (i.e. a measure of a quality of acommunication signal received at the destination apparatus is in balancewith a measure of a quality of communication signal received at the, oreach, intermediate apparatus). Thus, embodiments of the first aspect ofthe present invention may be advantageously used to maintain the qualityof the communication signal received by the destination apparatus at, ornear, the target value set by the destination apparatus. Thereafter, itmay be necessary for embodiments of a second aspect of the presentinvention to optimise the systems ensuring a balance is achieved betweenthe destination apparatus and the or each intermediate apparatus.

Thus, it should be appreciated that the indication deviation detectionmeans may be used in a system which has already been balanced, oroptimised. Thus, a deviation from the desired value, which may arise dueto an event which results in a change in a measure of a quality of acommunication signal at the destination apparatus will be detected, andthe required change the resource allocated to the previous intermediateapparatus determined.

If the indicator deviation is due to a change in the pathloss such thatthe quality of the communication signal received by the destinationdeviates from target, embodiments of the first aspect willadvantageously restore balance to the system by adjusting the transmitpower of the preceding intermediate apparatus. However, if the indicatordeviation is due to a change in the target quality set by thedestination apparatus, whilst embodiments of the first aspect may beadvantageously employed to adjust the transmit power at the intermediateapparatus in order that the new target can be attained, embodiments ofthe second aspect are needed in order to restore a balance bydetermining the corresponding change in transmit power for the othertransmitters in the multi-hop system.

Embodiments of the present invention may be implemented within awireless communication system employing any multiple access technique,including but not limited to: frequency division multiple access (FDMA),time division multiple access (TDMA) code division multiple access(CDMA) and orthogonal frequency division multiple access (OFDMA). In thecase of a CDMA system, in which all transmissions occur in the samefrequency band and each transmission is assigned a unique channelisationcode, the Gp factor represents the spreading factor or length of thecode used to spread the transmitted signal otherwise known as theprocessing gain. In the case of orthogonal spreading codes, up to Gpchannels are available for simultaneous transmission.

It should be appreciated that the term “user equipment” encompasses anydevice which is operable for use in a wireless communication system.Furthermore, although the present invention has been described primarilywith reference to terminology employed in presently known technology, itis intended that the embodiments of the present invention may beadvantageously applied in any wireless communication systems whichfacilitates the transmission of a communication signal between a sourceand destination, via an intermediate apparatus.

In any of the above aspects, the various features may be implemented inhardware, or as software modules running on one or more processors or acombination of these. The invention also provides operator programs(computer programs and computer program products) for carrying out anyof the methods described herein, and computer readable media havingstored thereon programs for carrying out any of the methods describedherein. A program embodying the invention may be stored on acomputer-readable medium, or it could, for example, be in the form of asignal such as a downloadable data signal provided from an Internet website, or it could be in any other form.

For a better understanding of the present invention and to show how thesame may be carried into effect, reference will now be made, by way ofexample, to the accompanying drawings in which:

FIG. 1A illustrates a single cell/relay model of a wirelesscommunication system;

FIG. 1B illustrates a two cell/relay model of a wireless communicationsystem;

FIGS. 2A and 2B each show a graphical representation of the theoreticalgain that may be achieved by a multi-hop communication system based onpathloss equation (A);

FIG. 3 illustrates an algorithm embodying the first aspect of thepresent invention;

FIG. 4 illustrates an algorithm embodying the second aspect of thepresent invention;

FIG. 5 illustrates parts of a base station embodying both the first andthe second aspects of the present invention;

FIG. 6 illustrates the relationship between source transmit power andintermediate transmit power in the case of a multi-hop communicationsystem having a non-regenerative relay node and using an FDD duplexingtechnique;

FIG. 7 illustrates the relationship between source transmit power andintermediate transmit power in the case of a multi-hop communicationsystem having a non-regenerative relay node and using a TDD duplexingtechnique;

FIGS. 8A and 8B illustrate the optimal NB transmit power as a functionof RN transmit power;

FIG. 9 shows a graphical illustration of the variation in the averagegain in throughput observed by users of a multi-hop system as comparedto that observed for a single hop system; and

FIG. 10 illustrate the optimal NB transmit power as a function of RNtransmit power where it is assumed that the communication link betweenthe source and destination apparatus has a 3 dB gain compared with theshorter multi-hop links.

An example of an algorithm which implements an embodiment of the firstaspect of the present invention will now be described with reference toFIG. 3 in which the source apparatus comprises a user equipment (UE),the intermediate apparatus comprises a relay node (RN) which is of thetype, and the destination apparatus comprises a base station (NB). Thebase station continually monitors the SINR and derives indicators of theSINR and the variation from target SINR.

The following sequence takes place following detection of a change in anindicator derived by the base station from a desired value in order todetermine a change in the transmit power of the intermediate apparatuswhich will tend bring the indicator derived by the destination apparatusback to said desired value.

The details of the algorithm are summarised as follows:

Uplink Algorithm 3: Part 1 Trigger: Periodically executed In NBAlgorithm Input Required by Origin Request for change in RN Changederived in NB and RN Transmit Power processed by RN Block in increase inRN Block is set/cleared in part 2 RN Transmit Power of the algorithmDestination & Signalling Algorithm Output Derivation Requirement Changein RN transmit Relative change Relative change derived at NB power andmade by RN

1. The base station detects a change in an indicator of SINR or in anindicator of the variation from target SINR such that the SINR at thedestination apparatus does not meet its target.

2. The determining means of the destination determines the requiredchange in the transmit power of the intermediate apparatus (RN).

3. A request is transmitted locally to a control means of thedestination apparatus for a change in the RN transmit power.

4. If the request is for a decrease in the RN transmit power, thecontrol means issues a command to the intermediate apparatus for adecrease in the transmit power of the RN.

5. If the request is for an increase in the RN transmit power, thecontrol means checks whether a prohibition, or block, is currently inplace which prohibits increases in the RN transmit power. Then:

5a. If it is determined that a prohibition is in place, the controlmeans ignores the request; or

5b. If it is determined that no prohibition is in place, the controlmeans issues a command to the intermediate apparatus; commanding anincrease in the transmit power of the RN.

6. The RN receives a command from the control means of the NB and checkswhether it can change its transmit power in accordance with the command.Then:

6a: If the RN determines that it cannot change its transmit power inaccordance with the command, it determines a revised change in transmitpower and adjusts its transmit power in accordance with the revisedtransmit power; or

6b: If the RN determines that it can change its transmit power inaccordance with the command, the RN changes its transmit poweraccordingly.

The algorithm described above will manage the case of the propagationloss varying between the RN and NB and the case of the NB modifying itstarget RSS or SINR. In order to handle the case of the propagation lossvarying between the UE and RN and the case that both the target in theNB and the propagation loss between the RN and NB varies, such that norequest for change in RN transmit power is generated, an algorithm whichimplements an embodiment of the second aspect of the present inventionoperates periodically as discussed below.

The details of the algorithm are summarised as follows:

Uplink Algorithm 3: Part 2 Trigger: Periodically executed in NBAlgorithm Input Required by Origin SINR at NB NB Known at NB SINR at RNNB Signalled from RN Destination & Signalling Algorithm OutputDerivation Requirement Change in UE transmit power Relative changeSignalled to UE via RN Block on RN power increase True/false test Part 1of the algorithm

This algorithm is executed periodically in addition to the algorithmdiscussed above with reference to FIG. 4. Alternatively, it is alsopossible for this algorithm to be implemented separately in a wirelessmulti-hop communication system.

The algorithm assumes that indicators of the SINR at the NB and RN arereported to the NB.

1. The NB monitors the indicators of the SINR from both the NB and RN.Then: 1a. if these are found to vary such that they are imbalanced, acontrol means of the NB determines the change in transmit power of theUE that is required to restore a balance in SINR; or

1b. If these are found to be balanced, a control means of the NB liftsany existing prohibition on an increase in the transmit power of the RN.

2. The control means issues a command to the UE, via the intermediateapparatus, commanding a change in the transmit power of the UE.

3. The UE receives the command from the NB and determines if it cancarry out the required change in transmit power. Then:

3a. If it is determined that the UE cannot carry out the requiredchange, the UE determines a revised change in transmit power and changesits transmit power in accordance with this revised change; or

3b. If it is determined that the UE can carry out the required change,the UE changes its transmit power in accordance with the requiredchange.

4. If the command issued by the control means was for a decrease in thetransmit power of the source apparatus, the control means lifts anyexisting prohibition on an increase in the transmit power of the RN.

5. If the command issued by the control means was for an increase intransmit power, the control means monitors the SINR indicator derived atthe intermediate apparatus to determine if the commanded change intransmit power of the source apparatus has been effected. Then:

5a. if it is determined that the change was not effected by the UE, thecontrol means places a prohibition on further increases in the transmitpower of the RN; or

5b. If it is determined that the change was effected by the UE, thecontrol means lifts any existing prohibition on an increase in thetransmit power of the RN.

FIG. 5 shows parts of a base station embodying the first and secondaspects of the present invention and comprises:

-   -   indicator derivation means (1), operable to derive one or more        indicators of the quality of a communication signal received at        the base station;indicator deviation detection means (2)        operable to detect a change in the, or one of the, indicators        derived by the base station; indicator receiving means (3),        operable to receive an indicator derived by the intermediate        apparatus;    -   imbalance detection means (4) operable to detect an imbalance        between an indicator derived by the indicator derivation means        and an indicator received by the indicator receiving means;    -   determining means (5), operable to determine a change in the        transmit power of the intermediate apparatus and/or a change in        the transmit power of the source apparatus, as the case may be,        following detection of an indicator deviation by the indicator        deviation detection means (3) and/or following detection of an        imbalance by the imbalance detection means (4); and    -   control means (6), operable to receive a request from the        determining means and, subject to various checks being performed        by the control means, to issue a command to the intermediate        apparatus and/or the source apparatus, as the case may be,        commanding a change in the transmit power of the intermediate        apparatus and/or the source apparatus respectively.

Following the detection of an imbalance and the issuance of a command tothe source apparatus for an increase in the transmit power of the sourceapparatus, the control means (6) is operable to prohibit an increase inthe transmit power of the intermediate apparatus (output) if it isdetected by the control means that the command for an increase in thetransmit power of the source apparatus was not met. Then, following thedetection of a change in the indicator derived by the base station, andprior to issuing a command to the intermediate apparatus for an increasein the transmit power of the intermediate apparatus, the control meansis operable to check (input) if any prohibition has been placed onincreases in the transmit power of the intermediate apparatus.

Theoretical Analysis

Although embodiments of the preset invention seek to balance the qualityindicators derived by the destination apparatus and intermediateapparatus without performing an explicitly calculation of the transmitpower required to achieve that balance, the following theoreticalanalysis, which derives possible solutions for explicitly calculatingthe optimal transmit power of the transmitting elements comprised in amulti-hop network for various deployment scenarios, is useful forunderstanding the present invention. Whilst the equations are developedsolely for the case of the connections that form the downlink in amulti-hop network, it is straightforward to adapt the equations derivedfor the case of the uplink. Such an adaptation is achieved by adoptingthe same methodology used to develop the expressions for the receivedSINR at the receiving nodes, where the transmitting nodes are now the UEand the RN and the receiving nodes are now the NB and RN. Onceexpressions for the SINR received at the RN and NB are arrived at, thesame methodology can be employed for each deployment scenario in orderto determine the optimal transmit power setting of the UE and RN. Foreach deployment scenario, theoretical solutions are obtained assuming asingle-cell model and a two-cell model. In the case of a two cell model,it is assumed that the deployment in both cells is identical and thatthe transmit powers on the bas station (BS) and the intermediateapparatus (I) are the same. It is also assumed that where appropriateP_(tx) _(—) _(tot,RN)=G_(p)P_(tx,RN) and P_(tx) _(—)_(tot,NB)=G_(p)P_(tx,NB) and that for the case of TDD both RN's transmitat the same time. This in effect generates the worse case scenario fortwo cells.

Theoretical solutions may be evolved from a consideration of thesignal-to-interference plus noise ratio (SINR) experienced by thereceiving nodes in a multi-hop system (i.e. the or each intermediateapparatus (I) and the destination apparatus (D)). The SINR at aparticular node is a measure of the quality of a communication signalreceived by that node and is a ratio of the received strength of thedesired signal to the received signal strength of the undesired signals(noise and interference).

As previously discussed, the considerations required for noise andinterference depend on the duplexing method used to separate signalreceived at an intermediate apparatus from those transmitted from anintermediate apparatus, the characteristics of the intermediateapparatus and also the level of inter-cell interference which is takeninto account (i.e. interference from neighbouring cells).

The following equation represents the SINR of a communication signalsent from an intermediate apparatus to a destination apparatus for allscenarios, where different terms may be ignored depending upon the typeof intermediate apparatus (e.g. non-regenerative or regenerative) andthe duplexing method:

${SINR}_{{RN} - {UE}} = \frac{G_{P}P_{{tx},{RN}}}{L_{{RN} - {UE}}\left( {N + \frac{P_{{tx},{RN}}}{L_{{RN} - {UE}}{SINR}_{{NB} - {RN}}} + \frac{P_{{tx\_ tot},{NB}}}{L_{{NB} - {UE}}}} \right)}$

For the case of FDD instead of TDD then the third term in the bracket isremoved and for the case of regenerative instead of non-regenerative thesecond term in the bracket is removed.

In the case of a two-cell model as illustrated in FIG. 1B, this becomes:

${SINR}_{{RN} - {UE}} = \frac{G_{P}P_{{tx},{{RN}\; 1}}}{L_{{{RN}\; 1} - {UE}}\begin{pmatrix}{N + \frac{P_{{tx},{{RN}\; 1}}}{L_{{{RN}\; 1} - {UE}}{SINR}_{{{NB}\; 1} - {{RN}\; 1}}} + \frac{P_{{tx\_ tot},{{NB}\; 1}}}{L_{{{NB}\; 1} - {UE}}} +} \\{\frac{P_{{tx\_ tot},{{NB}\; 2}}}{L_{{{NB}\; 2} - {UE}}} + \frac{P_{{tx\_ tot},{{RN}\; 2}}}{L_{{{RN}\; 2} - {UE}}}}\end{pmatrix}}$

The first three terms in the bracket in (2) are the same as those in(1). The additional last two terms originate from the interferenceexperienced from the neighbouring co-channel NB and RN respectively.Obviously if the neighbouring cell employs a different frequency or usesa different timeslot for relay transmission then the terms needed modelthis interference will vary. It should be appreciated that theseequations can be extended to a three-cell model or more for a higherlevel of accuracy.

Considering now the various possible deployment scenarios in turn, forthe case of DL transmissions transmitted between a base-station ornode-B (NB), via an intermediate relay node (RN) to a destination userequipment (UE).

1A. Regenerative relay with FDD—Single-cell model as illustrated in FIG.1A

In this case, the SINR at a destination UE which is connected to anintermediate RN is given by:

$\begin{matrix}{{SINR}_{{RN} - {UE}} = \frac{G_{p}P_{{tx},{RN}}}{L_{{RN} - {UE}}N}} & (1)\end{matrix}$

Where G_(p) is the processing gain, P_(tx,RN) is the transmit power onthe channel of interest at the RN, L_(RN-UE) is the propagation loss onthe NB to RN link and N is the noise. Note this assumes that nointra-cell interference exists.

The SINR at an intermediate RN which is operable to receive signals fromthe NB is given by:

$\begin{matrix}{{SINR}_{{NB} - {RN}} = \frac{G_{p}P_{{tx},{NB}}}{L_{{NB} - {RN}}N}} & (2)\end{matrix}$

Where P_(tx,NB) is the transmit power on the channel of interest at theNB and L_(NB-RN) is the propagation loss on the RN to UE link. Again, itis assumed that no intra-cell interference exists.

The overall throughput across the multi-hop link will be limited by thelower of the two SINR values as this will limit the rate at which datacan be transmitted to that entity. Any increase in transmit power thatcauses an SINR imbalance will not improve the performance of themulti-hop system; it will simply result in wasted energy and an increasein interference to any co-channel users.

Thus, assuming that the receiver at the intermediate RN and the receiverat the destination UE perform the same, then it follows that thetransmit power at the NB and RN should be set such that the SINR at theRN and UE is the same. Using this criterion for setting the ratio of thetransmit powers, it follows that the ratio is given by:

$\begin{matrix}{\frac{P_{{tx},{NB}}}{P_{{tx},{RN}}} = {\frac{L_{{NB} - {RN}}}{L_{{RN} - {UE}}} = \frac{b_{1}s_{1}^{n_{1}}}{b_{2}s_{2}^{n_{2}}}}} & (3)\end{matrix}$

Where b₁ and n₁ are the pathloss parameters for the NB to RN link whichis s₁ in length and b₂, n₂ and s₂ are associated with the RN to UE link.Thus using equation (3) it is possible to find either transmit powergiven the other.

1B. Regenerative relay with FDD—two cell model as shown in FIG. 1B

In this case, transmit power equations may be derived taking intoaccount interference caused by transmissions arising in the other cell.

In this case the SINR at a destination UE that is operable to receivesignals from an intermediate RN is now:

$\begin{matrix}{{SINR}_{{RN} - {UE}} = \frac{G_{p}P_{{tx},{RN}}}{L_{{RN} - {UE}}\left( {N + \frac{G_{p}P_{{tx},{RN}}}{L_{{RN} - {UE}}}} \right)}} & (4)\end{matrix}$

The optimal NB transmit power can be found by setting (4) and (2) to beequal. Therefore:

$\begin{matrix}\begin{matrix}{P_{{tx},{NB}} = \frac{L_{{NB} - {RN}}{NP}_{{tx},{RN}}}{L_{{RN} - {UE}}\left( {N + \frac{G_{p}P_{{tx},{RN}}}{L_{{RN} - {UE}}}} \right)}} \\{= \frac{L_{{NB} - {RN}}P_{{tx},{RN}}}{\left( {L_{{RN} - {UE}} + \frac{G_{p}P_{{tx},{RN}}}{N}} \right)}}\end{matrix} & (5)\end{matrix}$

(5) can be rearranged to find the intermediate RN transmit power giventhe source NB transmit power:

$\begin{matrix}{P_{{tx},{RN}} = \frac{L_{{RN} - {UE}}}{\left( {\frac{L_{{NB} - {RN}}}{P_{{tx},{NB}}} - \frac{G_{p}}{N}} \right)}} & (6)\end{matrix}$

2A. Regenerative relay with TDD: single cell model—FIG. 1A

It is assumed that the two links (source to intermediate, intermediateto destination) operate on the same frequency with TDD being used toseparate the receive and transmit operation of the RN (i.e. it is nolonger full duplex). If it is assumed that the timeslot in which the RNtransmits is not used by the NB then the equations described above forthe case of a regenerative relay with an FDD duplexing scheme can beused. However, if the source NB uses the same timeslot as theintermediate RN to communicate with apparatuses or nodes other than theNB, interference will result to the transmission made by the RN. In thiscase the SINR at a destination UE that is operable to receivecommunication signals from an intermediate RN is given by:

$\begin{matrix}\begin{matrix}{{SINR}_{{RN} - {UE}} = \frac{G_{p}P_{{tx},{RN}}}{L_{{RN} - {UE}}\left( {N + I} \right)}} \\{= \frac{G_{p}P_{{tx},{RN}}}{L_{{RN} - {UE}}\left( {N + \frac{P_{{tx\_ tot},{NB}}}{L_{{NB} - {UE}}}} \right)}}\end{matrix} & (7)\end{matrix}$

Where P_(tx) _(—) _(tot,NB) is the total transmission power from the NBand L_(NB-UE) is the propagation loss on the NB to UE link. In this casethe transmit power at the RN that ensures equal SINR is given by:

$\begin{matrix}{P_{{tx},{RN}} = {{P_{{tx},{NB}}\left( \frac{L_{{RN} - {UE}}}{L_{{NB} - {RN}}} \right)}\left( {1 + \frac{P_{{tx\_ tot},{NB}}}{{NL}_{{NB} - {UE}}}} \right)}} & (8)\end{matrix}$

Comparing equation (3) and equation (8) it is apparent that a simpleratio no longer yields the ideal balance. Assuming that P_(tx) _(—)_(tot,NB)=G_(p)P_(tx,NB) it is possible to write equation (8) as:

$\begin{matrix}\begin{matrix}{P_{{tx},{RN}} = {{P_{{tx},{NB}}\left( \frac{L_{{RN} - {UE}}}{L_{{NB} - {RN}}} \right)}\left( {1 + \frac{G_{p}P_{{tx},{NB}}}{{NL}_{{NB} - {UE}}}} \right)}} \\{= {\left( \frac{L_{{RN} - {UE}}}{L_{{NB} - {RN}}} \right)\left( {P_{{tx},{NB}} + \frac{G_{p}P_{{tx},{NB}}^{2}}{{NL}_{{NB} - {UE}}}} \right)}}\end{matrix} & (9)\end{matrix}$

From (9) it is possible to determine the ideal RN transmit power giventhe NB transmit power. It is worth noting that if the set-up of thesystem is arranged such that the second term in the second bracket isnegligible (i.e. P_(tx) _(—) _(tot,NB)/NL_(NB-UE)<<1) then the criteriondescribed above for the case of a regenerative relay with an FDD duplexscheme can be used.

It follows that the ideal NB transmit power given a certain RN transmitpower can be found from the roots of (9). Expressing (9) in thefollowing simplified form:

$\begin{matrix}{{{{\frac{L_{{RN} - {UE}}}{L_{{NB} - {RN}}}P_{{tx},{NB}}} + {\frac{L_{{RN} - {UE}}}{L_{{NB} - {RN}}}\frac{G_{p}}{{NL}_{{NB} - {UE}}}P_{{tx},{NB}}^{2}} - P_{{tx},{RN}}} = 0}{{{ax}^{2} + {bx} + c} = 0}} & (10)\end{matrix}$

Where x=P_(tx,NB),

${a = \frac{G_{p}L_{{RN} - {UE}}}{{NL}_{{NB} - {RN}}L_{{NB} - {UE}}}},{b = \frac{L_{{RN} - {UE}}}{L_{{NB} - {RN}}}}$

and c=−P_(tx,RN) it follows that the roots of (10) are given by:

$\begin{matrix}{x = \frac{{- b} \pm \sqrt{b^{2} - {4\; {ac}}}}{2\; a}} & (11)\end{matrix}$

As the transmit power is a positive number, only one root is defined, ittherefore follows that the optimal transmit power at the NB that ensuresequal SINR at the RN and UE is given by:

$\begin{matrix}{x = {P_{{tx},{NB}} = \frac{{- b} + \sqrt{b^{2} + {4\; {aP}_{{tx},{RN}}}}}{2a}}} & (12)\end{matrix}$

Finally, it is possible to use the definitions above to rewrite (9),which gives the optimal RN transmit power, in a similar simplified form:

P _(tx,RN) =bP _(tx,NB) +aP _(tx,NB) ²   (13)

2A. Regenerative relay with TDD: two-cell model as shown in FIG. 1B

In addition to assuming that the deployment in both is identical andthat the transmit powers on the NB and RN are the same, it is alsoassumed that where appropriate P_(tx) _(—) _(tot,RN)=G_(p)P_(tx,RN) andP_(tx) _(—) _(tot,NB)=G_(p)P_(tx,NB) and that for the case of TDD bothRN's transmit at the same time. This in effect generates the worse casescenario for two cells.

In this case the SINR at the destination UE that is operable to receivesignals from an intermediate RN is now:

$\begin{matrix}{{SINR}_{{RN} - {UE}} = \frac{G_{p}P_{{tx},{RN}}}{L_{{RN} - {UE}}\left( {N + \frac{2G_{p}P_{{tx},{NB}}}{L_{{NB} - {UE}}} + \frac{G_{p}P_{{tx},{RN}}}{L_{{RN} - {UE}}}} \right)}} & (14)\end{matrix}$

The optimal NB transmit power can be found by setting (14) and (2) to beequal:

$\begin{matrix}{{\frac{G_{p}P_{{tx},{NB}}}{{NL}_{{NB} - {RN}}} = \frac{G_{p}P_{{tx},{RN}}}{L_{{RN} - {UE}}\left( {N + \frac{2G_{p}P_{{tx},{NB}}}{L_{{NB} - {UE}}} + \frac{G_{p}P_{{tx},{RN}}}{L_{{RN} - {UE}}}} \right)}}{P_{{tx},{RN}} = {{P_{{tx},{NB}}\left( \frac{L_{{RN} - {UE}}}{L_{{NB} - {RN}}} \right)}\left( {1 + \frac{2P_{{tx\_ tot},{NB}}}{{NL}_{{NB} - {UE}}} + \frac{P_{{tx\_ tot},{RN}}}{{NL}_{{RN} - {UE}}}} \right)}}{{\left( \frac{L_{{RN} - {UE}}}{L_{{NB} - {RN}}} \right)\left( \frac{2G_{p}}{{NL}_{{NB} - {UE}}} \right)P_{{tx},{NB}}^{2}} + {\left( \frac{L_{{RN} - {UE}}}{L_{{NB} - {RN}}} \right)\left( {1 + \frac{G_{p}P_{{tx},{RN}}}{{NL}_{{RN} - {UE}}}} \right)P_{{tx},{NB}}} - P_{{tx},{RN}}}} & (15)\end{matrix}$

The optimal NB transmit power is found from the positive root of:

$\begin{matrix}{{{\left( \frac{L_{{RN} - {UE}}}{L_{{NB} - {RN}}} \right)\left( \frac{2G_{p}}{{NL}_{{NB} - {UE}}} \right)P_{{tx},{NB}}^{2}} + {\left( \frac{L_{{RN} - {UE}}}{L_{{NB} - {RN}}} \right)\left( {1 + \frac{G_{p}P_{{tx},{RN}}}{{NL}_{{RN} - {UE}}}} \right)P_{{tx},{NB}}} - P_{{tx},{RN}}} = 0} & (16)\end{matrix}$

Which is given by:

$\begin{matrix}{x = {P_{{tx},{NB}} = \frac{{- b} \pm \sqrt{b^{2} - {4\; {ac}}}}{2\; a}}} & (17)\end{matrix}$

Where in this case

${a = {\frac{2G_{p}}{{NL}_{{NB} - {UE}}}\frac{L_{{RN} - {UE}}}{L_{{NB} - {RN}}}}},{b = {\frac{L_{{RN} - {UE}}}{L_{{NB} - {RN}}}\left( {1 + \frac{G_{p}P_{{tx},{RN}}}{{NL}_{{RN} - {UE}}}} \right)}}$

and c=−P_(tx,RN), and both b and c are a function of the RN transmitpower.

Given the NB transmit power it is possible to rearrange (15) to find theRN transmit. It follows that the optimal RN transmit power is given by:

$\begin{matrix}{P_{{tx},{RN}} = \frac{{\left( {\frac{2G_{p}}{{NL}_{{NB} - {UE}}}\frac{L_{{RN} - {UE}}}{L_{{NB} - {RN}}}} \right)P_{{tx},{NB}}^{2}} + {\left( \frac{L_{{RN} - {UE}}}{L_{{NB} - {RN}}} \right)P_{{tx},{NB}}}}{1 - {\left( {\frac{G_{p}}{{NL}_{{RN} - {UE}}}\frac{L_{{RN} - {UE}}}{L_{{NB} - {RN}}}} \right)P_{{tx},{NB}}}}} & (18)\end{matrix}$

3A. Non-regenerative relay node (RN) with FDD—single cell model as shownin FIG. 1A

The difference between this case and that of a regenerative relay nodebeing used in conjunction with a FDD duplexing scheme is that the SINRat the UE is a function of the SINR at the RN, where the SINR at thedestination UE which is connected to the RN is given by:

$\begin{matrix}{{SINR}_{{RN} - {UE}} = \frac{G_{p}P_{{tx},{RN}}}{L_{{RN} - {UE}}\left( {N + \frac{P_{{tx},{RN}}}{L_{{RN} - {UE}}{SINR}_{{NB} - {RN}}}} \right)}} & (19)\end{matrix}$

The result is that the ideal balance is no longer derived from settingthe SINR at the UE equal to that at the RN. According to (19), the SINRat the RN needs to be set so that it does not prevent this target SINRat the UE from being obtained. However, the NB power must be controlledto limit the SINR at the RN rising beyond that practically required elseexcess interference and wasted transmit power will result.

FIG. 6 illustrates how the setting of NB and RN transmit power affectsthe SINR at the UE connected to the RN for a two different deploymentscenarios.

Thus, it can be seen that the optimal solution is to select the transmitpower of the NB and RN such that the system effectively operates on thediagonal fold in the surface shown in FIG. 6. It is possible to realisesuch a solution by taking the first derivative of (19) and finding thepoint at which increasing either the NB or RN transmit power results inminimal increase to SINR at UE.

In order to determine the first derivative of (19), it is rewritten as:

$\begin{matrix}\begin{matrix}{{SINR}_{{RN} - {UE}} = \frac{G_{p}P_{{tx},{RN}}}{L_{{RN} - {UE}}\left( {N + \frac{P_{{tx},{RN}}}{L_{{RN} - {UE}}\frac{G_{p}P_{{tx},{NB}}}{{NL}_{{NB} - {RN}}}}} \right)}} \\{= \frac{1}{\left( \frac{{NL}_{{RN} - {UE}}}{G_{p}P_{{tx},{RN}}} \right) + \left( \frac{{NL}_{{NB} - {RN}}}{G_{p}^{2}P_{{tx},{RN}}} \right)}}\end{matrix} & (20)\end{matrix}$

Defining y=SINR_(RN-UE),

$k_{1} = {{\frac{{NL}_{{RN} - {UE}}}{G_{p}}\mspace{14mu} {and}\mspace{14mu} k_{2}} = \frac{{NL}_{{NB} - {RN}}}{G_{p}^{2}}}$

it is possible to simplify (20) to be:

$\begin{matrix}{y = {\frac{1}{\frac{k_{1}}{P_{{tx},{RN}}} + \frac{k_{2}}{P_{{tx},{NB}}}} = \frac{P_{{tx},{NB}}}{\frac{k_{1}P_{{tx},{NB}}}{P_{{tx},{RN}}} + k_{2}}}} & (21)\end{matrix}$

In order to find the rate of change of SINR with P_(tx,NB) the quotientrule for differentiation is used:

$\begin{matrix}{\frac{y}{\left( P_{{tx},{NB}} \right)} = {\frac{k_{2}}{\left( {{\frac{k_{1}}{P_{{tx},{RN}}}P_{{tx},{NB}}} + k_{2}} \right)^{2}} = \nabla_{NB}}} & (22)\end{matrix}$

By solving (22) for P_(tx,NB) given the required gradient and P_(tx,RN)it is possible to find the optimal NB transmit power:

$\begin{matrix}{P_{{tx},{NB}} = \frac{P_{{tc},{RN}}\left( {\sqrt{\frac{k_{2}}{\nabla_{NB}}} - k_{2}} \right)}{k_{1}}} & (23)\end{matrix}$

In order to find the optimal RN transmit power given that of the NB, thedifferentiation of (21) is now performed with respect to P_(tx,RN). Inthis case the first order derivative is given by:

$\begin{matrix}{\frac{y}{\left( P_{{tx},{RN}} \right)} = {\frac{k_{1}}{\left( {{\frac{k_{2}}{P_{{tx},{NB}}}P_{{tx},{RN}}} + k_{1}} \right)^{2}} = \nabla_{RN}}} & (24)\end{matrix}$

And the optimal RN transmit power given that of the NB is:

$\begin{matrix}{P_{{tx},{RN}} = \frac{P_{{tc},{NB}}\left( {\sqrt{\frac{k_{1}}{\nabla_{RN}}} - k_{1}} \right)}{k_{2}}} & (25)\end{matrix}$

3B. Non-regenerative relay node (RN) with FDD—two cell model as shown inFIG. 1B

In a two cell model the SINR for the worse case of a destination UE atthe cell edge is given by:

$\begin{matrix}\begin{matrix}{{SINR}_{{RN} - {UE}} = \frac{G_{p}R_{{tx},{RN}}}{L_{{RN} - {UE}}\left( {N + \frac{P_{{tx},{RN}}}{L_{{RN} - {UE}}{SINR}_{{NB} - {RN}}} + \frac{G_{p}P_{{tx},{RN}}}{L_{{RN} - {UE}}}} \right)}} \\{= \frac{1}{\left( \frac{{NL}_{{RN} - {UE}}}{G_{p}P_{{tx},{RN}}} \right) + \left( \frac{{NL}_{{NB} - {RN}}}{G_{p}^{2}P_{{tx},{NB}}} \right) + 1}}\end{matrix} & (26)\end{matrix}$

Assuming that the transmit power of the two RN's is equal, thedeployment is identical across the two cells and that P_(tx) _(—)_(tot,RN)=G_(p)P_(tx,RN), then the simplified form of (26) is given by:

$\begin{matrix}{{SINR}_{{RN} - {UE}} = {\frac{1}{\frac{k_{1}}{P_{{tx},{RN}}} + \frac{k_{2}}{P_{{tx},{NB}}} + 1} = \frac{P_{{tx},{NB}}}{{\left( {\frac{k_{1}}{P_{{tx},{RN}}} + 1} \right)P_{{tx},{NB}}} + k_{2}}}} & (27)\end{matrix}$

The first derivative is now:

$\begin{matrix}{\frac{y}{\left( P_{{tx},{NB}} \right)} = \frac{k_{2}}{\left( {{\left( {\frac{k_{1}}{P_{{tx},{RN}}} + 1} \right)P_{{tx},{NB}}} + k_{2}} \right)^{2}}} & (28)\end{matrix}$

Thus the optimal NB transmit power can be found by:

$\begin{matrix}{P_{{tx},{NB}} = \frac{{P_{{tx},{RN}}\sqrt{\frac{k_{2}}{\nabla}}} - k_{2}}{k_{1} + P_{{tx},{RN}}}} & (29)\end{matrix}$

The optimal RN transmit power is found by taking the derivative of (27)with respect to P_(tx,RN):

$\begin{matrix}{\frac{y}{\left( P_{{tx},{RN}} \right)} = \frac{k_{1}}{\left( {{\left( {\frac{k_{2}}{P_{{tx},{NB}}} + 1} \right)P_{{tx},{RN}}} + k_{1}} \right)^{2}}} & (30)\end{matrix}$

Thus the optimal RN transmit power can be found by:

$\begin{matrix}{P_{{tx},{RN}} = \frac{{P_{{tx},{NB}}\sqrt{\frac{k_{1}}{\nabla}}} - k_{1}}{k_{2} + P_{{tx},{NB}}}} & (31)\end{matrix}$

4A—Non-regenerative relay with TDD—single cell model as shown in FIG. 1A

This case is similar to that described above for a non-regenerativeexcept for the fact that now interference from the NB must be taken intoaccount due to the fact that it transmits on the same frequency and atthe same time as the RN. In this case the SINR at the UE which isreceiving communication signals transmitted by the RN is given by:

$\begin{matrix}{{SINR}_{{RN} - {UE}} = \frac{G_{p}P_{{tx},{RN}}}{L_{{RN} - {UE}}\left( {N + \frac{P_{{tx},{RN}}}{L_{{RN} - {UE}}{SINR}_{{NB} - {Rn}}} + \frac{P_{{tx\_ tot},{NB}}}{L_{{NB} - {UE}}}} \right)}} & (32)\end{matrix}$

If the P_(tx,NB)/P_(tx,RN) is too large the SINR at the UE is limiteddue to insufficient RN transmit power and it is likely the area in whichthe link performance of a connection to a RN outperforms that for aconnection to the NB is reduced. Conversely, if it is too small then theSINR at the UE is limited by the low SINR at the RN.

In this case, the balance is even finer than of that described in thecase of a non-regenerative relay node employed in conjunction with anFDD duplexing scheme, as illustrated by FIG. 7. The optimal operatingpoint is given by finding the point at which the first derivative of(32) is equal to zero. In order to find this optimal point, (32) isfirst rearranged in the following form:

$\begin{matrix}\begin{matrix}{{SINR}_{{RN} - {UE}} = \frac{G_{p}P_{{tx},{RN}}}{L_{{RN} - {UE}}\left( {N + \frac{P_{{tx},{RN}}}{L_{{RN} - {UE}}\left( \frac{G_{p}P_{{tx},{NB}}}{{NL}_{{NB} - {RN}}} \right)} + \frac{P_{{tx\_ tot},{NB}}}{L_{{NB} - {UE}}}} \right)}} \\{= \frac{1}{\left( \frac{{NL}_{{RN} - {UE}}}{G_{p}P_{{tx},{RN}}} \right) + \left( \frac{{NL}_{{NB} - {RN}}}{G_{p}^{2}P_{{tx},{NB}}} \right) + \left( \frac{L_{{RN} - {UE}}P_{{tx},{NB}}}{L_{{NB} - {UE}}P_{{tx},{RN}}} \right)}}\end{matrix} & (33)\end{matrix}$

Defining y=SINR_(RN-UE),

$k_{1} = {{\frac{{NL}_{{RN} - {UE}}}{G_{p}}\mspace{14mu} {and}\mspace{14mu} k_{2}} = \frac{{NL}_{{NB} - {RN}}}{G_{p}^{2}}}$

Using the definitions from described in 3A above and

$k_{3} = \left( \frac{L_{{RN} - {UE}}}{L_{{NB} - {UE}}} \right)$

it is possible to simplify (33) to:

$\begin{matrix}\begin{matrix}{y = \frac{1}{\left( \frac{k_{1}}{P_{{tx},{RN}}} \right) + \left( \frac{k_{2}}{P_{{tx},{NB}}} \right) + \left( \frac{k_{3}P_{{tx},{NB}}}{P_{{tx},{RN}}} \right)}} \\{= \frac{P_{{tx},{NB}}}{{\left( \frac{k_{1}}{P_{{tx},{RN}}} \right)P_{{tx},{NB}}} + k_{2} + {\left( \frac{k_{3}}{P_{{tx},{RN}}} \right)P_{{tx},{NB}}^{2}}}}\end{matrix} & (34)\end{matrix}$

The next step is to find the single maxima of the parabolic function in(34) by solving:

$\begin{matrix}{\frac{y}{x} = 0} & (35)\end{matrix}$

Using the quotient rule to find the first derivative of (34):

$\begin{matrix}{\frac{y}{\left( P_{{tx},{NB}} \right)} = \frac{\begin{matrix}{{\frac{k_{1}}{P_{{tx},{RN}}}P_{{tx},{NB}}} + k_{2} + {\frac{k_{3}}{P_{{tx},{RN}}}P_{{tx},{NB}}^{2}} -} \\{P_{{tx},{NB}}\left( {\frac{k_{1}}{P_{{tx},{RN}}} + {\frac{2\; k_{3}}{P_{{tx},{RN}}}P_{{tx},{NB}}}} \right)}\end{matrix}}{\left( {{\frac{k_{1}}{P_{{tx},{RN}}}P_{{tx},{NB}}} + k_{2} + {\frac{k_{3}}{P_{{tx},{RN}}}P_{{tx},{NB}}^{2}}} \right)^{2}}} & (36)\end{matrix}$

The maxima of y is found by setting (36) equal to zero and solving forP_(tx,NB). It follows that the maximum SINR at the UE is obtained bysetting:

$\begin{matrix}{{{{\frac{k_{1}}{P_{{tx},{RN}}}P_{{tx},{NB}}} + k_{2} + {\frac{k_{3}}{P_{{tx},{RN}}}P_{{tx},{NB}}^{2}}} = {P_{{tx},{NB}}^{2}\left( {\frac{k_{1}}{P_{{tx},{RN}}} + {\frac{2\; k_{3}}{P_{{tx},{RN}}}P_{{tx},{NB}}^{2}}} \right)}}\mspace{79mu} {P_{{tx},{NB}} = \sqrt{\frac{P_{{tx},{RN}}k_{2}}{2\; k_{3}}}}} & (37)\end{matrix}$

Therefore, given the transmit power of the RN it is possible to use (37)to find the corresponding NB transmit power that ensures maximum SINR atthe UE that is connected to the RN.

For the case of finding the optimal RN transmit power given the NBtransmit power a similar approach to that described in above in the caseof a non-regenerative relay node employed in conjunction with an FDDduplexing scheme, can be used as the SINR at the UE is not a parabolicfunction of RN transmit power. In order to find the optimal RN transmitpower, (34) is rearranged to the following:

$\begin{matrix}\begin{matrix}{y = \frac{1}{\left( \frac{k_{1}}{P_{{tx},{RN}}} \right) + \left( \frac{k_{2}}{P_{{tx},{NB}}} \right) + \left( \frac{k_{3}P_{{tx},{NB}}}{P_{{tx},{RN}}} \right)}} \\{= \frac{P_{{tx},{RN}}}{\left( \frac{P_{{tx},{RN}}k_{2}}{P_{{tx},{NB}}} \right) + {k_{3}P_{{tx},{NB}}} + k_{1}}}\end{matrix} & (38)\end{matrix}$

The first derivative is now:

$\begin{matrix}{\frac{y}{\left( P_{{tx},{RN}} \right)} = {\frac{{k_{3}P_{{tx},{NB}}} + k_{1}}{\left( {\left( \frac{P_{{tx},{RN}}k_{2}}{P_{{tx},{NB}}} \right) + {k_{3}P_{{tx},{NB}}} + k_{1}} \right)^{2}} = \nabla}} & (39)\end{matrix}$

Solving (39) for P_(tx,RN) gives the optimal RN transmit power given theNB transmit power:

$\begin{matrix}{P_{{tx},{RN}} = \frac{P_{{tx},{NB}}\left( {\sqrt{\frac{{k_{3}P_{{tx},{NB}}} + k_{1}}{\nabla}} - \left( {{k_{3}P_{{tx},{NB}}} + k_{1}} \right)} \right)}{k_{2}}} & (40)\end{matrix}$

By observing the surface in FIG. 7 and from the form of (34) and theresult in (40) it is apparent that if the NB transmit power is smallthen the rate of change of SINR with RN transmit power will decreasewith increasing RN transmit power. However, for the case of large NBtransmit power, the SINR at the UE approximates to a linear function ofRN transmit power. The result is that in this case the solution to theproblem, as summarised in (40) will be infinite.

4B—Non-regenerative relay with TDD—two cell model as shown in FIG. 1B

The worse case, from the perspective of a UE at the cell edge, is whenthe neighbouring cell employs a TDD scheme with the same timeslot usedfor RN transmission. If it is assumed that the cells are equal in sizewith the same deployment and transmit power settings and that P_(tx)_(—) _(tot,RN/NB)=G_(p)P_(tx,RN/NB) then:

$\begin{matrix}\begin{matrix}{{SINR}_{{RN} - {UE}} = \frac{G_{p}P_{{tx},{RN}}}{L_{{RN} - {UE}}\begin{pmatrix}{N + \frac{P_{{tx},{RN}}}{L_{{RN} - {UE}}{SINR}_{{NB} - {R\; 1}}} +} \\{\frac{2\; G_{p}P_{{tx},{NB}}}{L_{{NB} - {UE}}} + \frac{G_{p}P_{{tx},{RN}}}{L_{{RN} - {UE}}}}\end{pmatrix}}} \\{= \frac{1}{\begin{matrix}{\left( \frac{{NL}_{{RN} - {UE}}}{G_{p}P_{{tx},{RN}}} \right) + \left( \frac{{NL}_{{NB} - {RN}}}{G_{p}^{2}P_{{tx},{NB}}} \right) +} \\{\left( \frac{2\; L_{{RN} - {UE}}P_{{tx},{NB}}}{L_{{NB} - {UE}}P_{{tx},{RN}}} \right) + 1}\end{matrix}}}\end{matrix} & (41)\end{matrix}$

In this case the simplified form of (4) is:

$\begin{matrix}\begin{matrix}{{SINR}_{{RN} - {UE}} = \frac{1}{\frac{k_{1}}{P_{{tx},{RN}}} + \frac{k_{2}}{P_{{tx},{NB}}} + {\frac{2\; k_{3}}{P_{{tx},{RN}}}P_{{tx},{NB}}} + 1}} \\{= \frac{P_{{tx},{NB}}}{{\left( {\frac{k_{1}}{P_{{tx},{RN}}} + 1} \right)P_{{tx},{NB}}} + k_{2} + {\frac{2\; k_{3}}{P_{{tx},{RN}}}P_{{tx},{NB}}^{2}}}}\end{matrix} & (42)\end{matrix}$

And the first derivative is:

$\begin{matrix}{\frac{y}{\left( P_{{tx},{NB}} \right)} = \frac{\begin{matrix}{{\left( {\frac{k_{1}}{P_{{tx},{RN}}} + 1} \right)P_{{tx},{NB}}} + k_{2} + {\frac{2\; k_{3}}{P_{{tx},{RN}}}P_{{tx},{NB}}^{2}} -} \\{P_{{tx},{NB}}\left( {\frac{k_{1}}{P_{{tx},{RN}}} + 1 + {\frac{4\; k_{3}}{P_{{tx},{RN}}}P_{{tx},{NB}}}} \right)}\end{matrix}}{\left( {{\left( {\frac{k_{1}}{P_{{tx},{RN}}} + 1} \right)P_{{tx},{NB}}} + k_{2} + {\frac{2\; k_{3}}{P_{{tx},{RN}}}P_{{tx},{NB}}^{2}}} \right)^{2}}} & (43)\end{matrix}$

Finally, the maxima is given by setting (43) equal to zero and solvingfor P_(tx,NB);

$\begin{matrix}{{{{\left( {\frac{k_{1}}{P_{{tx},{RN}}} + 1} \right)P_{{tx},{NB}}} + k_{2} + {\frac{2k_{3}}{P_{{tx},{RN}}}P_{{tx},{NB}}^{2}}} = {P_{{tx},{NB}}\left( {\frac{k_{1}}{P_{{tx},{RN}}} + 1 + {\frac{4\; k_{3}}{P_{{tx},{RN}}}P_{{tx},{NB}}}} \right)}}\mspace{79mu} {{k_{2} + {\frac{2\; k_{3}}{P_{{tx},{RN}}}P_{{tx},{NB}}^{2}}} = {\frac{4\; k_{3}}{P_{{tx},{RN}}}P_{{tx},{NB}}^{2}}}\mspace{79mu} {P_{{tx},{NB}} = \sqrt{\frac{P_{{tx},{RN}}k_{2}}{2\; k_{3}}}}} & (44)\end{matrix}$

In order to find the optimal RN transmit power given the NB transmitpower (42) is rearranged to:

$\begin{matrix}\begin{matrix}{y = \frac{1}{\frac{k_{1}}{P_{{tx},{RN}}} + \frac{k_{2}}{P_{{tx},{NB}}} + {\frac{2\; k_{3}}{P_{{tx},{RN}}}P_{{tx},{NB}}} + 1}} \\{= \frac{P_{{tx},{RN}}}{k_{1} + \frac{k_{2}P_{{tx},{RN}}}{P_{{tx},{NB}}} + {2\; k_{3}P_{{tx},{NB}}} + P_{{tx},{RN}}}}\end{matrix} & (45)\end{matrix}$

The first derivative is now:

$\begin{matrix}{\frac{y}{\left( P_{{tx},{RN}} \right)} = {\frac{k_{1} + {2\; k_{3}P_{{tx},{NB}}}}{\left( {k_{1} + {2\; k_{3}P_{{tx},{NB}}} + {P_{{tx},{RN}}\left( {1 + \frac{k_{2}}{P_{{tx},{NB}}}} \right)}} \right)^{2}} = \nabla}} & (46)\end{matrix}$

Solving (46) for P_(tx,RN) gives the optimal RN transmit power given theNB transmit power:

$\begin{matrix}{P_{{tx},{RN}} = \frac{{P_{{tx},{NB}}\sqrt{\frac{k_{1} + {2\; k_{3}P_{{tx},{NB}}}}{\nabla}}} - \left( {k_{1} + {2\; k_{3}P_{{tx},{NB}}}} \right)}{\left( {P_{{tx},{NB}} + k_{2}} \right)}} & (47)\end{matrix}$

Again, in the case of large NB transmit power, the SINR at the UZapproximates to a linear function of RN transmit power. The result isthat the solution to (47) will be infinite.

The optimal transmit power balance will now be determined based on thesolutions developed above for the different relay and duplexing schemesand for two separate deployment scenarios. These deployment scenariosare summarised in Table III and the propagation parameters of thepathloss equation in (48) are in Table IV.

L=b+10n log d   (48)

Where L is the pathloss in dB, b is in dB and is given in Table alongwith n, and d is the transmitter-receiver separation in metres.

TABLE III Deployment scenarios Scenario Parameter 1 2 Cell Radius 1867 mRelay Position 933 m 1400 m

The transmitter receiver separation is the same as the cell radius (i.e.the UE is located at the cell radius). The RN position quoted isrelative to the centre of the cell which is where the NB is located. TheRN positions are therefore the distance from the NB to the RN. The RN-UEis then the difference of the cell radius and the NB-RN separation.

TABLE V Propagation parameters. Link Parameter NB-UE NB-RN RN-UE b (dB)15.3 15.5 28 n 3.76 3.68 4

Regenerative Relay

Substituting the values given in Table III and Table IV into equations(3) and (5) for FDD and (12) and (17) for TDD it is possible to find theoptimal NB transmit power given the RN transmit power. FIG. 8A shows theoptimal NB transmit power as a function of RN transmit power for bothFDD and TDD for the two deployment scenarios.

Non-Regenerative Relay with FDD

Substituting the parameters into (23) and (24) it is possible to findthe optimal NB transmit power for the two deployment scenarios, as shownin FIG. 8B.

Non-Regenerative Relay with TDD

Substituting the parameters into (37) and (44) it is possible to findthe optimal NB transmit power for the two deployment scenarios, as shownin FIG. 8C.

System Level Simulation Results

System simulation of a multi-hop HSDPA network employingnon-regenerative relays with TDD duplexing with relays transmitting inevery third transmission time interval have been conducted in order tovalidate the predicted throughput gain based on results of FIG. 8C, withthe average packet call throughput gain being determined as the transmitpowers of the RN and NB are varied around the optimal point.

Results of a system level simulation for the two deployment scenariosdetailed above in Table III will now be presented. The simulationparameters are listed below in Table V and Table VI.

TABLE V Deployment parameters Parameter Value Base Station Inter-cellSeparation 2.8 km Sectors/cell 3 Antenna Height 15 m Antenna Gain 17 dBiRelay Station RN antenna 120° position ½ and ¾ cell radius Num/cell 9Antenna Height 5 m Antenna Gain 17 dBi User Number per sector 50Equipment Initial Distribution Random Velocity 3 km/h DirectionSemi-directed Update 20 m Traffic Models WWW

TABLE VI Simulation parameters Parameters Value Base Station/ HS-DSCHpower Variable Relay Node CPICH power 20% of total HARQ scheme ChaseHS-DSCH/frame 15 Relay buffer size  1.78 Mbits Ack/NAck Detection Errorfree NB Scheduler Round Robin Relay type Amplify & Forward User 10Equipment Thermal Noise Density −174 dBm/Hz Noise Figure 5 dBm DetectorMMSE

For both deployment scenarios the gain in the average packet callthroughput experienced by the users on that observed for the case of asingle hop system with NB transmission power of 30 dBm is plotted as afunction of NB transmit power for four different RN transmit powers.FIG. 9A shows the gain for deployment scenario 1 and FIG. 9B shows thegain for scenario 2.

Note that the channel gain for the NB to UE link was 3 dB higher thanfor the NB to RN and RN to UE link. This means that the interferenceexperienced by a UE connected to a RN from another NB is double thatused in the link analysis discussed above with reference to FIGS. 8A, 8Band 8C. The channel gain is due to the fact that a number of replicas ofthe transmitted signal are received, when the power on all these isadded it is found that for the case of the NB to UE channel the totalpower is double that on the NB to RN or RN to UE channel. This accountsfor the 3 dB gain, as 3 dB equates to double. As a result of the channelgain being higher for the NB to UE channel, this means that the receivedsignal power will be 3 dB (or double) higher than that used in theanalysis up to that point where no channel gain through multi-path wasconsidered.

Comparison of Link Based Prediction and System Simulation

FIG. 10 shows the optimal NB transmit power as a function of RN transmitpower for a non-regenerative relay for TDD for each deployment scenariowhere it is assumed the NB to UE link has a 3 dB gain compared with theother links. In this case, the predicted transmit power at the NB forthe RN transmit power used in the simulation are listed in Table VIIalong with the throughput gain that would be experienced if thesesettings were used and the maximum achievable.

TABLE IIII Predicted optimal NB transmit power and resulting simulatedthroughput gain that would have been achieved from this setting comparedwith the maximum gain observed. NB Transmit Power (dBm) & User PacketThroughput Gain Scenario 1 Scenario 2 RN Transmit Throughput ThroughputPower (dBm) Predicted Gain Max Gain Predicted Gain Max Gain 16 −0.5 33%40% 8.8 60% 67% 19 1 38% 43% 10.3 65% 74% 22 2.5 41% 46% 11.8 68% 74% 254 49% 51% 13.3 72% 75%

Table VII, FIG. 8A and FIG. 9B suggest that if power balancing isperformed according to a preferred embodiment of the present inventionusing a technique based on the equations developed above then theselected power balance will in general be in the region of the optimalpoint. In particular, for the transmit powers used the gain was shown toalways be within 10% of the achievable maximum, with the differencebeing due to shortcomings of using of a two-cell model to model amulti-cell system.

The necessity of transmit power balancing is apparent in the resultspresented in both FIG. 9A and FIG. 9B where it is shown that if the NBtransmit is increased beyond the optimal point then a significantdegradation in gain will be experienced despite the emission of moresignal energy. It also shows that if the NB transmit power is selectedcarefully then the sensitivity of the gain to RN transmit power isreduced.

1. A radio communication system including a mobile station, a relaystation, and a base station, wherein the mobile station transmits radiosignals to the relay station and the relay station relays the radiosignals to the base station, the communication system comprising a firstreception quality measuring unit configured to measure first receptionquality for a radio signal transmitted from the mobile station to therelay station; a second reception quality measuring unit configured tomeasure second reception quality for a radio signal transmitted from therelay station to the base station; a receiver configured to receive thefirst reception quality transmitted from the relay station to the basestation to collect the first reception quality and the second receptionquality by the base station; a first controller configure to generate afirst power control message and a second power control message takingthe collected first and the second reception quality into account andtending to bring the first and the second reception quality intobalance, by the base station as a centralized power control means for atotal transmits power management along a relay transmission path; afirst transmitter configured to transmit the first power control messageand the second power control message to the relay station from the basestation; a second controller configured to control transmission power ofradio signals to be transmitted from the relay station to the basestation based on the first power control message; a second transmitterconfigured to transmit the second power control message to the mobilestation to relay the second power control message; and a thirdcontroller configured to control transmission power of radio signals tobe transmitted from the mobile station to the relay station based on thesecond power control message.
 2. The radio communication systemaccording to claim 1, wherein the first reception quality measuring unitand the second reception quality measuring unit measure received signalstrength or signal to interference and noise ratio.
 3. The radiocommunication system according to claim 1, wherein the second controllercompares a measured result by the second reception quality measuringunit with a reference value and controls the transmission power toreduce a difference between the reference value and the measured resultby the reception quality measuring unit.
 4. The radio communicationsystem according to claim 1, wherein the second controller controls thetransmission power, by sending a control message from the base stationto the relay station.
 5. The radio communication system according toclaim 1, wherein the second controller controls the transmission powerwith taking capability of the relay station into account.
 6. Acommunication method for a radio communication system including a mobilestation, a relay station, and a base station, wherein the mobile stationtransmits radio signals to the relay station and the relay stationrelays the radio signals to the base station, the communication methodcomprising: measuring first reception quality for a radio signaltransmitted from the mobile station to the relay station; measuringsecond reception quality for a radio signal transmitted from the relaystation to the base station; receiving the first reception qualitytransmitted from the relay station to the base station to collect thefirst reception quality and a second reception quality by the basestation; generating a first power control message and a second powercontrol message taking the collected first and the second receptionquality into account and tending to bring the first and the secondreception quality into balance, by the base station centralized powercontrol means for a total transmission power management along a relaytransmission path; transmitting the first power control message and thesecond power control message to the relay station from the base station;controlling transmission power of radio signals to be transmitted fromthe relay station to the base station based on the first power controlmessage; transmitting the second power Control message to the mobilestation to relay the second power control message; and controllingtransmission power of radio signals to be transmitted from the mobilestation to the relay station based on the second power control message.7. A relay station which relays radio signals received from a mobilestation to a base station, the relay station comprising; a receptionquality measuring unit configured to measure first reception quality fora radio signal transmitted from the mobile station to the relay station;a transmitter configured to transmit the first reception quality to thebase station; a receiver configured to receive a first message andsecond message which are generated by the base station taking the firstreception quality and a second reception quality which is measured for aradio signal transmitted from the relay station to the base station intoaccount by the base station as a centralized power control means for atotal transmission power management along a relay transmission path; anda controller configured to control transmission power of radio signalsto be transmitted from the relay station to the base station based onthe first message and tending to bring the first and the secondreception quality into balance, and relay the second message to themobile station for transmission power control of radio signals to betransmitted to the mobile station to the relay station.
 8. A basestation used in a communication system including a mobile station and arelay station which relays radio signals received from the mobilestation to the base station, the base station comprising: a receiverconfigured to receive first reception quality of a radio signal which ismeasured for a radio signal transmitted from the mobile station to therelay station; and a reception quality measuring unit configured tomeasure second reception quality for a radio signal transmitted from therelay station to the base station, wherein said base station collectsthe first reception quality and the second reception quality, said basestation further comprising a controller configured to generate a firstmessage and a second message taking the collected first and the secondreception quality into account and tending to bring the first and thesecond reception quality into balance, by the base station as acentralized power control cans for a total transmission power managementalong a relay transmission path; and a transmitter configured totransmit the first message and the second message to the relay station,wherein the first message is used for transmission power control of atransmission signal to be transmitted from the relay station to the basestation and the second message is relayed to the mobile station by therelay station for transmission power control of a transmission signal tobe transmitted from the mobile station to the relay station.